1. Field of the Invention
The present invention relates to a vacuum circuit breaker and, more particularly, to a contact structure for such a breaker.
2. Description of the Prior Art
Generally, the vacuum circuit breakers have three basic requirements which must be met. The first of these is that the circuit breaker must be capable of momentarily carrying current and closing against momentary current loads substantially in excess of the rated current capacity of the breaker without producing objectionable weld spots between the contacts of the breaker and without otherwise damaging the contacts of the breaker. The second requirement is that the breaker must be capable of breaking a current when overloaded. The third requirement is that the circuit breaker must be capable of withstanding, without damage or a disruptive discharge, an impulse crest voltage and a continuous A.C. voltage at the rated voltage of the device. J. W. Porter et al., in U.S. Pat. No. 3,497,652, have proposed that the contacts of a vacuum breaker be formed of an alloy consisting of copper - aluminum - bismuth. Vacuum breakers which have contacts of these alloys can interrupt high inductive currents at rated voltages, can carry currents and close against such currents without producing objectionable contact-welds between the contacts, and can successfully withstand high impulse crest voltages of at least 95 KV and continuous 60 cycle voltages of at least 36 KV r.m.s. when the contacts are fully separated. Although such contacts are entirely satisfactory for many circuit applications, vacuum breakers whose contacts are formed of contact making and breaking regions of copper - aluminum - bismuth alloys as disclosed in the above-described patent have not been able to meet certain mechanical requirements. For instance, a mechanical property of vacuum breakers relates to the tendency of contacts to fracture by the application of an external mechanical force which is applied many times thereby interrupting the operation of the device through transient operations as well as when a thermal force is imparted to the contacts and contact base when a device containing such contacts is used in welding processes thereby being subjected to heat ranging from 600.degree.-800.degree. C. Contact containing devices also encounter high temperatures of 400.degree.-600.degree. C. employed in baking processes. Also, mechanical stress is imparted to such contact containing devices when cooled. Moreover, during manufacture of the vacuum breakers, the contacts exhibit the embrittlement phenomenon which reduces the eutectic modification of the alloy from the .beta. phase to the .alpha.+.gamma. phase as the central structural feature of the natural alloy. The .alpha.+.gamma. phase contains a compound which embrittles the .gamma. phase and which is coarse and reaches a length of 1000 .mu.m and a width more than 100 .mu.m.
A need, therefore, continues to exist for an alloy material for vacuum circuit breakers whose contact points exhibit favorable mechanical characteristics.